Hello!
This bug is actually very easy to reproduce, it has probably not been reported yet because it’s kind of an unusual thing to do. If you add a checkbox as a child to a button widget in UMG, it appears darker than normally. My checkbox with a custom style is even completely black. The same effect can also be seen when adding a named slot widget to a button, the font appears black instead of white like normally.
The reason I need the check box to be a child of the button is that they can both have keyboard focus and be hovered at the same time, so wrapping both with an overlay for example isn’t an option, but wrapping the check box in certain other widgets like a retainer box seems to be a workaround.